Forgive me if im wrong, but didn't MS just recently "sell" it's 10th million unit (in March), where sell means actually shipped to stores, not sold?
I got the figures from here but yes, your correct that it seems the current figure is about 10m shipped which actually doesn't tally up with the quartely figures for some reason.
For the record PC sales in 2006 were about 230m I believe with more than half of those being desktops.
So if anything its probably more like 3.5% of total PC sales in 2006. Or if we discount the laptop market completely and assume it made up half of all PC sales then you are looking at about 7% of all desktops sold last year being equivilent to 360 sales.